Two: Not everyone has read new codex
Three: Perhaps psykers lower leadership, so what. I'm making my callidus appear on their side of the table, flame, assault independent charecter heavy weapon teams farseers whatever
Two: I think everyone who is discussing about units in the new codex (this thread) have read at least the confirmed rumours OR at least _should_ have read.
Three: Im not sure I unerstand what you mean right here. The shredder becomes a monster with psyker squad. Template, auto hit, no cover, 2+ wounding and ap1. The animus speculum comes pretty close, but it gives covers, is only S5 and you have to hit with it.
Although its an easy pick after you start comparing the other abilities. The Culexus basically has couple of useless/situational abilities against enemy psykers and one ability that lowers the ld of your psyker squad to 7. The Callidius has phase sword, poison blades and jump pack wich are all ok abilities although a bit situational as I dont think he should get to cc if you are using him with psykers. Then he has two really good abilities also, "polymorphine" and especially the "word in your ear"-ability as you can get your opponenets marines/firewarriors out of cover turn 1 and start battlecannoning them.
@all:Anyway the psyker battle squad is so powerful alone do you think he even nees the assasin ? Its really fragile for its cost.
Also if you are taking the assasin what would you do with the inquisitor ? I was thinking inferno pistol and shove him in a chimera with my vets. You could also give him the "divine pronouncement" , but that would just be overkill on top of overkill.